Driving apparatus and semiconductor device

ABSTRACT

A driving apparatus is provided, the driving apparatus including: a gate driving unit that drives a semiconductor element; a sampling unit that samples, in an on-period of the semiconductor element, an observation value that changes according to an on-current flowing through the semiconductor element; and a changing unit that changes a driving condition under which the gate driving unit drives a gate of the semiconductor element when the semiconductor element is turned off according to the observation value sampled in an on-period of the semiconductor element.

The contents of the following Japanese patent application areincorporated herein by reference:

NO. 2017-243058 filed in JP on Dec. 19, 2017.

BACKGROUND 1. Technical Field

The present invention relates to a driving apparatus and a semiconductordevice.

2. Related Art

Conventionally, various techniques for turning off a semiconductorelement while at the same time reducing turn-off loss, surge voltage orthe like have been proposed for a driving apparatus that drives the gateof the semiconductor element (please see Patent Documents 1 to 9, forexample). Such techniques change driving conditions for example whenvoltage of a semiconductor element to be turned off reached apower-supply voltage.

Patent Document 1: Japanese Patent No. 5516705

Patent Document 2: Japanese Patent Application Publication No.2015-204659

Patent Document 3: Japanese Patent No. 4742828

Patent Document 4: Japanese Patent No. 6168253

Patent Document 5: Japanese Patent Application Publication No.2008-78816

Patent Document 6: Japanese Patent Application Publication No.2009-195017

Patent Document 7: Japanese Patent Application Publication No.2008-193717

Patent Document 8: Japanese Patent Application Publication No.2014-176228

Patent Document 9: Japanese Patent Application Publication No.2016-77110

However, with increase in the switching speed of a semiconductorelement, it has been becoming difficult to change driving conditions bydetecting an abnormality based on measured values of voltage or the likefrom the start to the end of turn-off.

SUMMARY

In order to solve the above-mentioned drawbacks, a first aspect of thepresent invention may provide a driving apparatus. The driving apparatusmay include a gate driving unit that drives a semiconductor element. Thedriving apparatus may include a sampling unit that samples, in anon-period of the semiconductor element, an observation value thatchanges according to an on-current flowing through the semiconductorelement. The driving apparatus may include a changing unit that changesa driving condition under which the gate driving unit drives a gate ofthe semiconductor element when the semiconductor element is turned offaccording to the observation value sampled in an on-period of thesemiconductor element.

The sampling unit may hold the sampled observation value. The changingunit may change a driving condition under which the gate driving unitdrives the gate when the semiconductor element is turned off accordingto the held observation value.

The sampling unit may sample the observation value in a turn-on periodof the semiconductor element.

The sampling unit may sample the observation value corresponding to alength of time starting at a start of a turn-on period of thesemiconductor element and terminating when an element voltage of thesemiconductor element becomes a reference voltage.

The sampling unit may have a comparator circuit that compares an elementvoltage of the semiconductor element with a reference voltage

The sampling unit may have a sampling circuit that samples theobservation value corresponding to a result of comparison by thecomparator circuit at timing at which first reference time has elapsedafter a start of a turn-on period of the semiconductor element.

The sampling unit may sample the observation value corresponding to agate voltage or gate current of the semiconductor element at timing atwhich second reference time has elapsed after a turn-on period of thesemiconductor element has started.

The sampling unit may sample the observation value corresponding to agate voltage or gate current of the semiconductor element in a mirrorperiod in a turn-on period of the semiconductor element.

The changing unit may be able to switch to a first driving condition anda second driving condition under which a charge amount of a gate of thesemiconductor element is changed gradually as compared to the firstdriving condition. If the observation value sampled in an on-period ofthe semiconductor element indicates an on-current equal to or lower thana reference current, the changing unit may maintain a driving conditionfor the gate driving unit at the first driving condition in a turn-offperiod of the semiconductor element. If the observation value sampled inan on-period of the semiconductor element indicates an on-currentexceeding a reference current, the changing unit may switch a drivingcondition for the gate driving unit from the first driving condition tothe second driving condition during a turn-off period of thesemiconductor element.

The reference current may be a value obtained by subtracting apredetermined margin from an overcurrent level at which a surge voltagethat occurs accompanying turn-off of the semiconductor element becomesan overvoltage.

The driving apparatus may further include a timeout detecting unit thatdetects whether or not an on-period of the semiconductor element hasexceeded predetermined third reference time. If an on-period of thesemiconductor element has exceeded the third reference time, thechanging unit may switch a driving condition for the gate driving unitfrom the first driving condition to the second driving condition duringa turn-off period of the semiconductor element irrespective of amagnitude of the observation value.

The semiconductor element may be a wide-bandgap semiconductor element.

A second aspect of the present invention may provide a semiconductordevice. The semiconductor device may include the driving apparatusaccording to the first aspect of the present invention. Thesemiconductor device may include the semiconductor element.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a semiconductor device according to an embodiment.

FIG. 2 shows one example of operation waveforms of the semiconductordevice according to the embodiment.

FIG. 3 shows another example of operation waveforms of the semiconductordevice according to the embodiment.

FIG. 4 shows a semiconductor device according to a variant of theembodiment.

FIG. 5 shows one example of operation waveforms of the semiconductordevice according to the variant.

FIG. 6 shows another example of operation waveforms of the semiconductordevice according to the variant.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, (some) embodiment(s) of the present invention will bedescribed. The embodiment(s) do(es) not limit the invention according tothe claims, and all the combinations of the features described in theembodiment(s) are not necessarily essential to means provided by aspectsof the invention.

FIG. 1 shows a semiconductor device 1 according to the presentembodiment. In the figure, the outline arrow indicates voltage.

The semiconductor device 1 corresponds to one phase of a powerconverting device used for driving a motor or supplying power, as oneexample, and outputs voltage, from a power-supply output terminal 105,converted by switching connection between a positive side power line 101and a negative-side power line 102, and the power-supply output terminal105.

Here, a DC voltage Ed of 600 to 800V, for example, is applied betweenthe positive side power line 101 and the negative-side power line 102.In addition, wiring inductances 1011, 1021 corresponding to the wirelengths of the positive side power line 101 and negative-side power line102, respectively, may exist in them.

The semiconductor device 1 includes: a semiconductor element 11 and asemiconductor element 12; a driving apparatus 2 associated with thesemiconductor element 11 on the positive side; and a driving apparatus 5associated with the semiconductor element 12 on the negative side.Because the configuration of the driving apparatus 2 on the positiveside is the same as the driving apparatus 5 on the negative side, anexplanation thereof is omitted.

The semiconductor element 11 and semiconductor element 12 aresequentially connected in series between the negative-side power line102 and the positive side power line 101. The power-supply outputterminal 105 may be connected at the middle point between thesemiconductor element 11 and the semiconductor element 12.

The semiconductor element 11 and semiconductor element 12 are switchelements which are switchingly turned on and off by the drivingapparatus 2 and the driving apparatus 5, respectively. As one example,the semiconductor element 11 and semiconductor element 12 constitute theupper arm and lower arm of the power converting device.

At least one of the semiconductor element 11 and the semiconductorelement 12 may be a wide-bandgap semiconductor element. The wide-bandgapsemiconductor element refers to a semiconductor element having a bandgaplarger than that of a silicon semiconductor element, and for example isa semiconductor element containing SiC, GaN, diamond, a galliumnitride-based material, a gallium oxide-based material, AlN, AlGaN, ZnOor the like. The wide-bandgap semiconductor element can improve aswitching speed more than a silicon semiconductor element can.

In addition, in the present example, the semiconductor element 11 andsemiconductor element 12 are MOSFETs, and have parasitic diodes whosepositive side power line 101 sides are cathodes. (illustrated in FIG.1). Semiconductor elements with other structures such as IGBTs orbipolar transistors can be applied to the semiconductor element 11 andsemiconductor element 12, and a diode, a Schottky barrier diode or thelike is connected in anti-parallel with each of the semiconductorelements as necessary.

The driving apparatus 5 drives the semiconductor element 12 based on aninput signal. For example, the driving apparatus 5 cooperates with thedriving apparatus 2, and when turning on the semiconductor element 11and the semiconductor element 12 alternately, turns off one of theelements, and thereafter turns on the other element. The drivingapparatus 5 reduces turn-off loss and additionally suppresses the surgevoltage by switching the variable speed of gate charge of thesemiconductor element 12 in a turn-off period (in the presentembodiment, a period from the start to the completion of execution ofturn-off, as one example), that is, by switching the variable speed ofgate voltage (Vgs) which is a gate-source voltage of the semiconductorelement 12.

The driving apparatus 5 includes a gate driving unit 6, a sampling unit7 and a changing unit 8. The gate driving unit 6 drives thesemiconductor element 12. For example, the gate driving unit 6 suppliesthe gate of the semiconductor element 12 with a turn-on signal to turnon the semiconductor element 12. In addition, the gate driving unit 6supplies, through the changing unit 8, the gate of the semiconductorelement 12 with a turn-off signal to turn off the semiconductor element12.

The sampling unit 7 samples, in an on-period of the semiconductorelement 12, an observation value that changes according to an on-currentflowing through the semiconductor element 12. For example, the samplingunit 7 samples an observation value in a turn-on period of thesemiconductor element 12.

Here, the sampled observation value indicates an on-current of thesemiconductor element 12, and is used for predicting whether or not anovercurrent due to the surge voltage occurs in a turn-off period. Thisis because if an operation waveform in a turn-on period, an on-currentand an operation waveform in a turn-off period thereafter are examinedpreliminarily, it is possible to predict whether an on-current of thesemiconductor element 12 is large or small, and eventually whether ornot an overcurrent occurs in a turn-off period based on whether a lengthof time until an observation value becomes a reference value in aturn-on period is long or short or whether an observation value at thetime point when a reference length of time has elapsed is large orsmall. In addition, this is because it is possible to predict whether ornot an overcurrent occurs in a turn-off period based on a current at thetime of turn-on because particularly if PWM-control is being performed,currents almost match at the time of turn-on and turn-off.

The sampling unit 7 supplies the changing unit 8 with the sampledobservation value. The sampling unit 7 has a comparator circuit 70 and asampling circuit 71.

The comparator circuit 70 compares an element voltage of thesemiconductor element 12, a drain-source voltage as one example, in thepresent embodiment, with a reference voltage Vref. The reference voltageVref of the element voltage may correspond to a reference current ofon-current of the semiconductor element 12. This reference current maybe a value at an overcurrent level at which the surge voltage thatoccurs accompanying turn-off of the semiconductor element 12 becomes anovervoltage, but in the present embodiment, is a value obtained bysubtracting a predetermined margin from the value at the overcurrentlevel. As one example, when the overcurrent level at which the surgevoltage becomes an overvoltage is 400 A, the reference current may be300 A.

The comparator circuit 70 supplies the sampling circuit 71 with a resultof the comparison. As one example, in the present embodiment, thecomparator circuit 70 has a resistance type potential divider 700, areference voltage source 701 and a comparator 702.

The resistance type potential divider 700 divides an element voltage ofthe semiconductor element 12. The resistance type potential divider 700may have a resistance 700A provided between the drain terminal of thesemiconductor element 12 and the comparator 702, and a resistance 700Bprovided between a portion between the resistance 700A and thecomparator 702, and the source terminal of the semiconductor element 12.

The reference voltage source 701 supplies the comparator 702 with thereference voltage V_(ref). As one example, in the present embodiment,the reference voltage source 701 is connected between the comparator 702and the source terminal of the semiconductor element 12.

The comparator 702 compares an element voltage obtained by dividing anelement voltage of the semiconductor element 12 at the resistance typepotential divider 700 with the reference voltage Vref. As one example,in the present embodiment, the comparator 702 supplies the samplingcircuit 71 with a signal that becomes “1” if the element voltage isequal to or higher than the reference voltage Vref and becomes “0” ifthe element voltage is lower than the reference voltage Vref.

At the timing at which first reference time has elapsed after the startof a turn-on period of the semiconductor element 12, the samplingcircuit 71 samples an observation value corresponding to a result ofcomparison by the comparator circuit 70. Thereby, an observation valuecorresponding to a length of time starting at the start of a turn-onperiod of the semiconductor element 12 and terminating when the elementvoltage of the semiconductor element 12 becomes the reference voltageVref is sampled. Although as one example, in the present embodiment, thesampling circuit 71 samples, as an observation value, a value output bythe comparator circuit 70, it may sample a value that changes accordingto a value output by the comparator circuit 70.

Here, the turn-on period start timing may be the timing at which anon-signal is supplied to the gate of the semiconductor element 12, ormay be the timing at which a gate voltage or gate current startsvarying. The first reference time end timing, that is, the samplingtiming, may be the timing at which an observation value that enablesprediction as to whether or not an overcurrent will be generated duringa turn-off period is acquired, and may be before the semiconductorelement 12 enters a steady on-state. The length of the first referencetime is adjusted such that the surge voltage does not reach anovervoltage level. For example, a length of time starting at the turn-onperiod start timing and terminating when an output of the comparator 702becomes “1” when the smallest on-current that makes the surge voltage atthe time of turn-off becomes an overvoltage may be preliminarilymeasured and used as the length of the first reference time.

The sampling circuit 71 supplies the changing unit 8 with the sampledobservation value. As one example, in the present embodiment, thesampling circuit 71 has a pulse generating circuit 710, a logical NOTcircuit 711, a logical AND circuit 712 and a hold circuit 713.

The pulse generating circuit 710 supplies the logical NOT circuit 711with a pulse signal having the length of the first reference time. Asone example, the pulse generating circuit 710 supplies the logical NOTcircuit 711 with a signal that becomes “1” for the first reference timeaccording to an on-signal being output from the gate driving unit 6, andbecomes “0” before and after the first reference time.

The logical NOT circuit 711 performs a NOT operation on a pulse signalfrom the pulse generating circuit 710, and supplies the logical ANDcircuit 712 with a result of the operation. Thereby, the logical NOTcircuit 711 supplies the logical AND circuit 712 with a signal thatbecomes “0” for the first reference time according to an on-signal beingoutput from the gate driving unit 6 and becomes “1” before and after thefirst reference time.

The logical AND circuit 712 performs a logical AND operation on thesignal supplied from the logical NOT circuit 711 and the on-signalsupplied from the gate driving unit 6 to the gate of the semiconductorelement 12. Thereby, the logical AND circuit 712 outputs a signal thatbecomes “1” at the timing at which the first reference time has elapsedafter the start of a turn-on period in a period during which theon-signal is supplied to the semiconductor element 12. The logical ANDcircuit 712 supplies the trigger terminal of the hold circuit 713 with asignal indicating a result of the operation.

The hold circuit 713 holds an observation value. The hold circuit 713samples an observation value at rising timing at which a result of theoperation of logical AND by the logical AND circuit 712 becomes “1”.Thereby, an observation value indicating whether or not an elementvoltage is lower than the reference voltage Vref at the timing at whichthe first reference time has elapsed after the start of a turn-on periodin a period during which the on-signal is supplied to the semiconductorelement 12 is sampled. As one example, in the present embodiment, if theobservation value sampled at this timing is “1”, an on-current exceedingthe reference current is indicated, and it is anticipated that anovercurrent will be generated at the time of turn-off. If theobservation value is “0”, an on-current equal to or lower than thereference current is indicated, and it is anticipated that anovercurrent will not be generated at the time of turn-off. The holdcircuit 713 supplies the changing unit 8 with the held observationvalue.

The hold circuit 713 may reset the held observation value after thecompletion of turn-off of the semiconductor element 12. As one example,the hold circuit 713 may reset it upon the element voltage of thesemiconductor element 12 being the DC voltage Ed, may reset it upon theelapse of a required length of time from the start of supply of anoff-signal to the semiconductor element 12 until completion of turn-off,or may reset it upon an on-signal being supplied to the semiconductorelement 11. However, if the hold circuit 713 updates the heldobservation value every time sampling is performed, it may not be reset.

The changing unit 8 changes a driving condition applied to driving ofthe gate by the gate driving unit 6 when the semiconductor element 12 isturned off, according to an observation value sampled in an on-period ofthe semiconductor element 12, as one example, in the present embodiment,an observation value held by the hold circuit 713. The changing unit 8has a logical AND circuit 80, a timer circuit 81, an off-conditionchanging circuit 82 and a timeout detecting unit 83.

The logical AND circuit 80 performs a logical AND operation on theobservation value supplied from the hold circuit 713 and the off-signalsupplied from the gate driving unit 6 to the gate of the semiconductorelement 12. Thereby, the logical AND circuit 80 outputs a signal tobecome “1” in a period during which the observation value from the holdcircuit 713 becomes “1” in a period during which an off-signal issupplied to the semiconductor element 12. The logical AND circuit 80supplies the timer circuit 81 with a signal indicating a result of theoperation.

The timer circuit 81 supplies the off-condition changing circuit 82 withtiming to change a driving condition. For example, the timer circuit 81supplies the off-condition changing circuit 82 with a signal of “1”instructing a change upon or after the elapse of reference switchingtime after a signal of “1” is supplied from the logical AND circuit 80.The timer circuit 81 supplies the off-condition changing circuit 82 witha signal of “0” not instructing a change if it is not being suppliedwith a signal of “1” from the logical AND circuit 80, for example if anobservation value supplied from the hold circuit 713 is “0” and it isanticipated that an overcurrent will not be generated at the time ofturn-off. The end timing of the reference switching time, that is, thedriving condition change timing, may be the end timing, or timingtherearound, of a mirror period in a turn-off period of thesemiconductor element 12, for example.

The timer circuit 81 may be reset after completion of turn-off of thesemiconductor element 12. As one example, the timer circuit 81 may bereset upon the hold circuit 713 being reset, and set an output signal to“0”.

The off-condition changing circuit 82 switches a driving conditionapplied when the semiconductor element 12 is turned off between a firstdriving condition and a second driving condition according to a signalfrom the timer circuit 81. For example, the off-condition changingcircuit 82 may change a driving condition applied to turn-off from thefirst driving condition to the second driving condition upon a signal of“1” being supplied from the timer circuit 81. Thereby, if an observationvalue sampled at the sampling circuit 71 is “1” and it is indicated thatan on-current exceeds the reference current, a driving condition isswitched from the first driving condition to the second drivingcondition during a turn-off period of the semiconductor element 12. Inaddition, if a sampled observation value is “0” and it is indicated thatan on-current is equal to or lower than the reference current, a drivingcondition for the gate driving unit 6 is maintained at the first drivingcondition in a turn-off period of the semiconductor element 12.

Here, under the second driving condition, a charge amount of the gate ofthe semiconductor element 12 is changed gradually compared to under thefirst driving condition. Under the second driving condition, theoff-condition changing circuit 82 corrects and supplies a gate signal atthe time of turn-off so that the speed of charge injection to the gateof the semiconductor element 12 is lowered as compared to under thefirst driving condition. For example, the off-condition changing circuit82 may lower gate current at the time of turn-off, may lower gatevoltage at the time of turn-off or may stop supply of gate current atthe time of turn-off. To lower the gate current at the time of turn-off,for example, the internal path through which the gate current flows maybe switched from a path in which the resistance value is low to a pathin which the resistance value is high. To lower the gate voltage at thetime of turn-off, for example, the power-supply voltage of a gatedriving circuit may be switched to a voltage value lower than a normalvalue. Alternatively, the value of power-supply voltage (not illustratedin figures) to be supplied to the driving apparatus 5 may be switched toa low voltage value. Voltage and current of a turn-off signal here referto, but are not limited to, gate voltage and gate current, respectively.

The timeout detecting unit 83 detects whether or not an on-period of thesemiconductor element 12 has exceeded predetermined third referencetime. In addition, regardless of a signal supplied from the timercircuit 81, if the on-period of the semiconductor element 12 hasexceeded the third reference time, the timeout detecting unit 83 causesthe off-condition changing circuit 82 to change a driving condition tothe second driving condition during a turn-off period of thesemiconductor element 12. Thereby, the driving condition applied in theturn-off period of the semiconductor element 12 is switched from thefirst driving condition to the second driving condition regardless ofthe magnitude of an observation value. The third reference time may betime long enough such that it is anticipated that an overcurrent will begenerated in a turn-off period, and as one example, may be time longerthan a switching cycle predetermined for the semiconductor device 1. Inaddition, the third reference time may be time in which an on-current atthe time of turn-on is estimated to change to be equal to or larger thana tolerated value until the time of turn-off. The timing for the timeoutdetecting unit 83 to switch a driving condition may be timing at whichthe above-mentioned reference switching time has elapsed after anoff-signal is supplied from the gate driving unit 6. Although in thepresent embodiment, the on-period is defined as a period starting at thestart of supply of a turn-on signal of the semiconductor element 12 orthe start of a change in gate voltage of the semiconductor element 12and terminating at the end of supply of a turn-on signal from the gatedriving unit 6 to the semiconductor element 12 or the start of supply ofa turn-off signal, definitions of both the time of start and the time ofend of an on-period are not limited to them.

According to the above-mentioned semiconductor device 1, a drivingcondition applied to the gate at the time of turn-off is changedaccording to an observation value sampled in an on-period, as oneexample, an observation value sampled and held in an on-period.Accordingly, the driving condition can be changed according to abreaking current at the time of turn-off predicted from the observationvalue sampled in an on-period. Therefore, unlike the case where anobservation value is measured in a turn-off period and a drivingcondition is changed, the driving condition can be surely changed in aturn-off period even if the on-off switching speed is high. Accordingly,it is possible to prevent the surge voltage due to turn-off frombecoming an overvoltage. In addition, a turn-off period can be shortenedand turn-off loss can be reduced compared to the case where turn-off isperformed using only a driving condition under which a gate chargeamount is changed gradually and without changing the driving condition.

In addition, because the driving condition is changed to one under whicha gate charge amount is changed gradually if an observation valueindicates that there is an on-current exceeding the reference current,it is possible to surely prevent the surge voltage generated due toturn-off from becoming an overvoltage.

In addition, because a value obtained by subtracting a predeterminedmargin from an overcurrent level at which an overvoltage is generated isused as the reference current, it is possible to more surely prevent anovervoltage due to turn-off.

In addition, if an on-period of the semiconductor element 12 hasexceeded the third reference time, the driving condition applied to thegate is switched to the second driving condition during a turn-offperiod of the semiconductor element 12 regardless of the magnitude of anobservation value. Accordingly, it is possible to surely prevent anovervoltage if it is likely that the surge voltage due to turn-offbecomes an overvoltage because of a long on-period.

FIG. 2 shows one example of operation waveforms of the semiconductordevice 1. Based on operation waveforms of FIG. 2, the semiconductordevice 1 changes a driving condition applied at the time of turn-off bypredicting an overvoltage of the surge voltage occurring at the time ofturn-off of the semiconductor element 12.

First, an on-signal to be high at and after the time point t1 is outputfrom the gate driving unit 6 (please see the waveform of the on-signal).The on-signal is supplied to the gate of the semiconductor element 12,the pulse generating circuit 710 and the logical AND circuit 712.

When the on-signal is supplied, the pulse generating circuit 710supplies the logical NOT circuit 711 with a pulse signal having thelength of first reference time T1 (please see the waveform of the pulsesignal). Thereby, the logical NOT circuit 711 outputs a signal thatbecomes “0” for the first reference time T1 (please see the waveform ofthe NOT signal). In this example, the end timing of the first referencetime T1 is assumed to be the time point t4.

In addition, in response to supply of the on-signal, the gate voltage ofthe semiconductor element 12 rises at and after the time point t2 whichis delay time Δt1 after the time point t1 and exceeds the gate thresholdvalue Vth (5 v, as one example) at the time point t3, and thesemiconductor element 11 starts being turned on (please see the waveformof the gate voltage). Thereby, the element voltage lowers, and theelement current increases (please see the waveforms of the elementvoltage and element current). Because in this operation example, theelement voltage becomes lower than the reference voltage Vref at thetime point t5 which is later than the end timing of the first referencetime T1, a value output by the comparator circuit 70 is “1” until thetime point t5 and becomes “0” at the time point t5 (please see thewaveform of a comparison result).

Next, at the time point t4 at which the first reference time T1 elapses,an output from the pulse generating circuit 710 becomes “0”, and anoutput from the logical NOT circuit 711 becomes “1” (please see thewaveforms of the pulse signal and NOT signal). Thereby, an output fromthe logical AND circuit 712 becomes “1” at the time point t4, and avalue “1” output by the comparator circuit 70 at this time point t4 issampled and held as an observation value by the hold circuit 713 (pleasesee the waveform of the observation value). The hold circuit 713supplies the logical AND circuit 80 with the held observation value “1”.

Next, after the output of the on-signal from the gate driving unit 6ends, an off-signal to be high at and after the time point t6 is output(please see the waveforms of the on-signal and off-signal). Theoff-signal is supplied to the logical AND circuit 80 and off-conditionchanging circuit 82.

When the off-signal is supplied, at and after the time point t7 which isdelay time Δt2 after the time point t6, the gate voltage of thesemiconductor element 12 lowers, and the element voltage rises (pleasesee the waveforms of the gate voltage and element voltage).

In addition, because when the off-signal is supplied, the observationvalue “1” is being supplied from the hold circuit 713 to the logical ANDcircuit 80, a signal of “1” is input to a timer circuit as a result of alogical AND operation (please see the waveform of the timer input).Thereby, the timer circuit supplies the off-condition changing circuit82 with a signal of “1” instructing to change a driving condition at orafter the time point t8 at which the reference switching time T0 elapsesafter the time point t6 (please see the waveform of the timer output).Then, as a result of switching by the off-condition changing circuit 82of a driving condition for the semiconductor element 12 at and after thetime point t8 to the second driving condition, change in the gate chargeamount of the semiconductor element 12 becomes gradual one, and thesurge voltage accompanying turn-off is suppressed.

In this operation example, the hold circuit 713 and timer circuit 81 arereset at the time point t9 after turn-off of the semiconductor element12 is completed (please see the waveforms of the observation value andtimer output).

FIG. 3 shows another example of operation waveforms of the semiconductordevice 1. The semiconductor device 1 performs turn-off without changinga driving condition if an overvoltage due to the surge voltage is notpredicted at the time of turn-off as shown in the operation waveforms inFIG. 3. Because the operation at and after the time point t1 and beforeand at the time point t3 is the same as that in FIG. 2, an explanationthereof is omitted.

In this operation example, the element voltage becomes lower than thereference voltage Vref at the time point t5′ before the time point t4which is the end timing of the first reference time T1 (please see thewaveform of the element voltage). Because of this, the value output bythe comparator circuit 70 is “1” until the time point t5′ and becomes“0” at the time point t5′ (please see the waveform of the comparisonresult).

Next, at the time point t4 at which the first reference time T1 elapses,an output from the pulse generating circuit 710 becomes “0”, and anoutput from the logical NOT circuit 711 becomes “1” (please see thewaveforms of the pulse signal and NOT signal). Thereby, an output fromthe logical AND circuit 712 becomes “1” at the time point t4, and avalue “0” output by the comparator circuit 70 at this time point t4 issampled and held as an observation value by the hold circuit 713 (pleasesee the waveform of the observation value). The hold circuit 713supplies the logical AND circuit 80 with the held observation value “0”.Because of this, in this operation example, the driving condition changetiming is not supplied from the timer circuit 81 to the off-conditionchanging circuit 82, and a change is not made in the driving conditionat the time of turn-off.

FIG. 4 shows a semiconductor device 1A according to a variant of thepresent embodiment. At least one of driving apparatuses 2A, 5A in thesemiconductor device 1A, both the driving apparatuses 2A, 5A in thepresent variant, changes or change a driving condition during theturn-off period using an observation value corresponding to the gatevoltage or gate current. Because the configuration of the drivingapparatus 2A on the positive side is the same as the driving apparatus5A on the negative side, an explanation thereof is omitted.

The driving apparatus 5A has a comparator circuit 70A and a samplingcircuit 71A. The comparator circuit 70A compares the gate voltage of thesemiconductor element 12 with the reference voltage VrefA using acomparator 702A. The reference voltage VrefA corresponds to thereference current of an on-current of the semiconductor element 12, andis supplied to the comparator 702A by the reference voltage source 701A.The comparator circuit 70A may compare the gate current with thereference gate current, instead of comparing the gate voltage with thereference voltage VrefA.

The sampling circuit 71A has a pulse generating circuit 710A and a holdcircuit 713A. The pulse generating circuit 710A supplies the holdcircuit 713A with a pulse signal having the length of the secondreference time from the start of a turn-on period of the semiconductorelement 12. The hold circuit 713A samples an observation value at thefalling timing of the pulse signal. Thereby, at the timing at which thesecond reference time has elapsed after the start of a turn-on period ofthe semiconductor element 12, an observation value corresponding to agate voltage of the semiconductor element 12 is sampled.

As one example, the pulse generating circuit 710A supplies the holdcircuit 713A with a signal that becomes “1” for the second referencetime according to an on-signal being output from the gate driving unit6, and becomes “0” before and after the second reference time. The endtiming of the second reference time may be in a mirror period in aturn-on period of the semiconductor element 12. Thereby, an observationvalue corresponding to the mirror voltage at the time of turn-on of thesemiconductor element 12 is sampled by the hold circuit 713A.

Because according to the above-mentioned semiconductor device 1A, anobservation value corresponding to the gate voltage is sampled, it isnot necessary to make the withstand voltage of the comparator circuithigh, unlike the case where an observation value corresponding to anelement voltage or element current is sampled. Accordingly theconfiguration of the comparator circuit can be simplified.

In addition, an observation value corresponding to a gate voltage issampled in a mirror period in which the gate voltage becomes constant,setting of the sampling timing can be made easy to perform.

FIG. 5 shows one example of operation waveforms of the semiconductordevice 1A. Based on operation waveforms of FIG. 5, the semiconductordevice 1A changes a driving condition applied at the time of turn-off bypredicting an overvoltage of the surge voltage occurring at the time ofturn-off of the semiconductor element 12.

First, an on-signal to be high at and after the time point t11 is outputfrom the gate driving unit 6 (please see the waveform of the on-signal).The on-signal is supplied to the gate of the semiconductor element 12and the pulse generating circuit 710A.

When the on-signal is supplied, the gate voltage of the semiconductorelement 12 rises at and after the time point t12 which is delay time Δt1after the time point t1 and exceeds the gate threshold value (notillustrated in the figure), and the semiconductor element 11 startsbeing turned on (please see the waveform of the gate voltage). Then,because the gate voltage exceeds the reference voltage VrefA at the timepoint t13, a value output by the comparator circuit 70A is “0” until thetime point t13, and becomes “1” at the time point t13 (please see thewaveform of the comparison result).

In addition, in response to supply of the on-signal, the pulsegenerating circuit 710A supplies the hold circuit 713A with a pulsesignal having the length of second reference time T2 (please see thewaveform of the pulse signal). In this example, the end timing of thesecond reference time T2 is at the time point t14 in the mirror periodduring which the gate voltage is maintained at the mirror voltage Vm,and the time point t14 is later than the time point t13. Because ofthis, the value “1” output by the comparator circuit 70 is sampled andheld as an observation value by the hold circuit 713A at the time pointt14 (please see the waveform of the comparison result and observationvalue). The hold circuit 713A supplies the logical AND circuit 80 withthe held observation value “1”.

Next, after the output of the on-signal from the gate driving unit 6ends, an off-signal to be high at and after the time point t16 is output(please see the waveforms of the on-signal and off-signal). Theoff-signal is supplied to the logical AND circuit 80 and off-conditionchanging circuit 82.

When the off-signal is supplied, at and after the time point t17 whichis delay time Δt2 after the time point t16, the gate voltage of thesemiconductor element 12 lowers, and the element voltage rises (pleasesee the waveforms of the gate voltage and element voltage).

In addition, because when the off-signal is supplied, the observationvalue “1” is supplied from the hold circuit 713A to the logical ANDcircuit 80, a signal of “1” is input to a timer circuit as a result of alogical AND operation, (please see the waveform of the timer input).Thereby, the timer circuit supplies the off-condition changing circuit82 with a signal of “1” instructing to change a driving condition at orafter the time point t18 at which the reference switching time T0elapses after the time point t16 (please see the waveform of the timeroutput). Then, as a result of switching by the off-condition changingcircuit 82 of a driving condition for the semiconductor element 12 atand after the time point t18 to the second driving condition, change inthe gate charge amount of the semiconductor element 12 becomes gradualone, and the surge voltage accompanying turn-off is suppressed.

In this operation example, the hold circuit 713A and timer circuit 81are reset at the time point t19 after turn-off of the semiconductorelement 12 is completed (please see the waveforms of the observationvalue and timer output).

FIG. 6 shows another example of operation waveforms of the semiconductordevice 1A. The semiconductor device 1A performs turn-off withoutchanging a driving condition if an overvoltage due to the surge voltageis not predicted at the time of turn-off as shown in the operationwaveforms in FIG. 6. Because the operation at and after the time pointt11 and before and at the time point t13 is the same as that in FIG. 5,an explanation thereof is omitted.

In this operation example, at the time point t13′ later than the timepoint t14, the gate voltage exceeds the reference voltage Vref (pleasesee the waveform of the gate voltage), and a value output by thecomparator circuit 70A becomes “1” (please see the waveform of thecomparison result). Because of this, when the second reference time T2elapses at the time point t14 and an output from the pulse generatingcircuit 710 becomes “0” (please see the waveform of the pulse signal), avalue “0” output by the comparator circuit 70A is sampled and held as anobservation value by the hold circuit 713A (please see the waveform ofthe observation value). Therefore, because the observation value “0” issupplied to the logical AND circuit 80, and driving condition changetiming is not supplied from the timer circuit 81 to the off-conditionchanging circuit 82, a change is not made to a driving condition at thetime of turn-off.

In the above-mentioned embodiment and variant explained, a result ofcomparison by the comparator circuit 70 is sampled and held by the holdcircuit 713 according to a signal supplied to a trigger terminal.However, the semiconductor devices 1, 1A may perform sampling andholding using another configuration. For example, the semiconductordevice 1 may perform a logical AND operation on a signal output by thecomparator circuit 70, a signal output by the logical NOT circuit 711and an on-signal from the gate driving unit 6 and hold a result of theoperation. Likewise, the semiconductor device 1A may perform a logicalAND operation on a signal output by the comparator circuit 70A, aninverted signal output by the pulse generating circuit 710A and anon-signal from the gate driving unit 6, and hold a result of theoperation.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated by “prior to,” “before,” or the like and as longas the output from a previous process is not used in a later process.Even if the process flow is described using phrases such as “first” or“next” in the claims, embodiments, or diagrams, it does not necessarilymean that the process must be performed in this order.

What is claimed is:
 1. A driving apparatus comprising: a gate drivingunit that drives a semiconductor element; a sampling unit that samples,in an on-period of the semiconductor element, an observation value thatchanges according to an on-current flowing through the semiconductorelement; a changing unit that changes a driving condition under whichthe gate driving unit drives a gate of the semiconductor element whenthe semiconductor element is turned off according to the observationvalue sampled in an on-period of the semiconductor element; and atimeout detecting unit that detects whether or not an on-period of thesemiconductor element has exceeded a predetermined reference time,wherein if an on-period of the semiconductor element has exceeded thepredetermined reference time, the changing unit switches a drivingcondition for the gate driving unit from a first driving condition to asecond driving condition during a turn-off period of the semiconductorelement irrespective of a magnitude of the observation value.
 2. Thedriving apparatus according to claim 1, wherein the sampling unit holdsthe sampled observation value, and the changing unit changes a drivingcondition under which the gate driving unit drives the gate when thesemiconductor element is turned off according to the held observationvalue.
 3. The driving apparatus according to claim 1, wherein thesampling unit samples the observation value in a turn-on period of thesemiconductor element.
 4. The driving apparatus according to claim 3,wherein the sampling unit samples the observation value corresponding toa length of time starting at a start of a turn-on period of thesemiconductor element and terminating when an element voltage of thesemiconductor element becomes a reference voltage.
 5. The drivingapparatus according to claim 3, wherein the sampling unit has: acomparator circuit that compares an element voltage of the semiconductorelement with a reference voltage; and a sampling circuit that samplesthe observation value corresponding to a result of comparison by thecomparator circuit at timing at which first reference time has elapsedafter a start of a turn-on period of the semiconductor element.
 6. Thedriving apparatus according to claim 3, wherein the sampling unitsamples the observation value corresponding to a gate voltage or gatecurrent of the semiconductor element at timing at which second referencetime has elapsed after a turn-on period of the semiconductor element hasstarted.
 7. The driving apparatus according to claim 6, wherein thesampling unit samples the observation value corresponding to a gatevoltage or gate current of the semiconductor element in a mirror periodin a turn-on period of the semiconductor element.
 8. The drivingapparatus according to claim 1, wherein the changing unit can switch toa first driving condition and a second driving condition under which acharge amount of a gate of the semiconductor element is changedgradually as compared to the first driving condition, if the observationvalue sampled in an on-period of the semiconductor element indicates anon-current equal to or lower than a reference current, a drivingcondition for the gate driving unit is maintained at the first drivingcondition in a turn-off period of the semiconductor element, and if theobservation value sampled in an on-period of the semiconductor elementindicates an on-current exceeding a reference current, a drivingcondition for the gate driving unit is switched from the first drivingcondition to the second driving condition during a turn-off period ofthe semiconductor element.
 9. The driving apparatus according to claim1, wherein the semiconductor element is a wide-bandgap semiconductorelement.
 10. A semiconductor device comprising: the driving apparatusaccording to claim 1; and the semiconductor element.
 11. A drivingapparatus comprising: a gate driving unit that drives a semiconductorelement; a sampling unit that samples, in an on-period of thesemiconductor element, an observation value that changes according to anon-current flowing through the semiconductor element; and a changingunit that changes a driving condition under which the gate driving unitdrives a gate of the semiconductor element when the semiconductorelement is turned off according to the observation value sampled in anon-period of the semiconductor element, wherein the changing unit canswitch to a first driving condition and a second driving condition underwhich a charge amount of a gate of the semiconductor element is changedgradually as compared to the first driving condition, if the observationvalue sampled in an on-period of the semiconductor element indicates anon-current equal to or lower than a reference current, a drivingcondition for the gate driving unit is maintained at the first drivingcondition in a turn-off period of the semiconductor element, and if theobservation value sampled in an on-period of the semiconductor elementindicates an on-current exceeding a reference current, a drivingcondition for the gate driving unit is switched from the first drivingcondition to the second driving condition during a turn-off period ofthe semiconductor element, the driving apparatus further comprises: atimeout detecting unit that detects whether or not an on-period of thesemiconductor element has exceeded predetermined third reference time,wherein if an on-period of the semiconductor element has exceeded thethird reference time, the changing unit switches a driving condition forthe gate driving unit from the first driving condition to the seconddriving condition during a turn-off period of the semiconductor elementirrespective of a magnitude of the observation value.